Ultrastructural study of poorly differentiated medullary carcinoma of the thyroid.

Five cases of sporadic medullary carcinoma of the thyroid (MTC) with rapidly progressive disease were studied ultrastructurally. The tumour cells had poorly differentiated C cell characteristics. They exhibited smaller secretory granules in their narrow cytoplasm. Morphometric analysis disclosed that the average diameter of the secretory granules of the cases with a poor prognosis was 173.0 nm in comparison with 254.2 nm of well differentiated cases. The granules were fewer in the poor prognostic group (1.31/mu 2) than the well differentiated group (2.75/mu 2). Increased free ribosomes and polysomes were noted in the cytoplasm and dispersed chromatin in the nuclei. These cases should be therefore classified as poorly differentiated MTC rather than atypical or anaplastic MTC.